Conventionally, emissions requirements regulate constituents of tailpipe out emissions. Specifically, the amount of nitrogen oxide released into the atmosphere is regulated. Engines may include after treatment systems to mitigate the amount of nitrogen oxide being released into the atmosphere. The after treatment systems typically include a nitrogen oxide sensor to determine the amount of nitrogen oxide present in an exhaust stream.
However, occasionally the nitrogen oxide sensor may fail. Failure may occur due to various factors such as, for example, overheating, manufacturing defects, and the like. Typically, during operation, an output of the nitrogen oxide sensor should directly correspond to the amount of fuel used by the engine. That is, when more fuel is used by the engine, generally, more nitrogen oxide should be sensed by the nitrogen oxide sensor. Hence, the failure of the nitrogen oxide sensor can be determined based on monitoring the fuel usage and the sensed nitrogen oxide content of the exhaust stream.
For example, U.S. Published Application No. 2004/0010364 discloses an apparatus for detecting a failure of an exhaust gas sensor disposed downstream of a catalyst converter in an exhaust manifold. The apparatus comprises a control unit. The control unit determines a ratio between an amplitude value of a first output of the exhaust gas sensor and an amplitude value of a second output of an air-fuel ratio sensor. The air fuel ratio sensor is disposed upstream of the catalyst converter. The control unit detects a failure of the exhaust gas sensor based on the ratio. In one embodiment, a statistical process using a successive least squares method is applied to the ratio. The control unit detects a failure of the exhaust gas sensor based on the statistically processed ratio. In another embodiment, the statistical process is applied to both the output of the exhaust gas sensor and the output of the air-fuel ratio sensor. The control unit detects a failure of the exhaust gas sensor based on a ratio between the statistically processed outputs of the exhaust gas sensor and the air-fuel ratio sensor.